I can't say much that will help, as that I have never Raced the Dez or anything and I've never been on a WR but here it goes. The YZ more than likely could be equipped with a larger tank, and will have an awesome hard hitting powerband. Any tight situations that it gets itself into, it can get out. The lighter weight can be a major advantage at that too. If I did it I'd go with not only a lowering link, but better suited tires (since it is a track bike), a larger tank, and quite possibly a new exaust system if there are any regs/to hard hitting powerband. As for the WR it is going to be closer to what he will use, but it will be alot heavier and have a duller powerband. Him already being a little short for the bike means it needs to be lighter, the extra weight will tire you out attemping to handle on a larger bike.
If you were to ask me, "what should I get" I'd go with the YZ and spend a little garage time on it getting it perfect. Plus if he ever decideds to reach out into other fields of the sport, his bike is already able to handle the swap, where the WR can't.